1. Field of the Invention
The present invention relates to an electric switching unit of the type which comprises the following components:
a body,
an electromagnet having a stationary yoke, a moving armature and a stationary excitation coil, said electromagnet being associated with said body by means of one of its stationary elements;
a plurality of switches each comprising a stationary contact member and a moving contact which are provided with contact-studs, the moving contacts being placed together with their pressure springs on an insulating contact-holder which is capable of displacement in sliding motion within said body in order to cause opening and closing of the switches and which is mechanically associated with said moving armature whilst the stationary contacts are rigidly fixed to the body;
a gas-tight protective casing which is fixed on the body so as to contain the switches and is provided with insulation with respect to said switches;
resilient means capable of placing the moving armature in a rest position in which it is located at a distance from the yoke.
2. Description of the Prior Art
Switching units of this type can be employed either when the environment in which they are placed has an aggressive character (acidity, humidity, dust particles) or else when it is desired to reduce the harmful and inevitable influence of oxygen on the contacts, even when the surrounding atmosphere is normal.
Other practical considerations may also entail the need to confine the switches in special atmospheres. Of primary interest here are the design objectives which potentially offer a reduction in equipment construction costs by dispensing with the use of silver in the contact-studs of these switches.
It is indeed well-known that the present widespread use of silver or silver alloys in contact-studs arises mainly from the fact that the oxides of this metal are not stable. Since these oxides decompose in the vicinity of 200.degree. C., pure silver is commonly found in the contact region of studs in which it is employed, with the result that their use and operation in an oxidizing atmosphere is accordingly justified.